Device at fork truck

ABSTRACT

Springing for support wheels ( 5 ) of low lifting trucks arranged below the forks comprising a common spring device arranged in the transfer mechanism of the lifting movement to the support wheels ( 5 ). The spring device includes a cylinder ( 13 ), a piston ( 14 ), a spring package of cup springs ( 16 ) and an extending push rod that in the cylinder is connected a ring shaped element ( 17 ) located between the spring package and the cylinder end ( 19 ) through which the push rod extend. The piston is arranged in the other end of the spring package and the cylinder is via a return valve coupled in parallel with the load lifting hydraulic system of the truck so that the pressure in this pretensions the ring shaped element ( 17 ) towards the cylinder end proportionally to the load, so that springing does not take place until the dynamics forces overcome the pretension.

[0001] This invention is related to the type of low-lifting fork trucksthat are used to move loading pallets with load in the floor plane. Thistype of trucks is for instance used for the transferring of loads fromlorries into hyper-markets or the like and in particular for reloadingof freighted gods to and from or between different long distancelorries. In particular the latter use is increasing since the aim is tostore the merchandise in between as seldom as possible and instead aswift continuous flow all the way from maker or producer to the finalbuyer or user is desired. The desired swiftness in the reloading thusnot only lead to an increased use of this type of trucks but also to anincreased demand on their speed. The speeds are already today such thatin some cases safety belts are required for the drivers, this in orderto avoid injuries at collisions etcetera, as well as to avoid that thedriver falls off. The increased speed further means that the damages onthe transported gods increase, the maintenance costs of the trucksincrease and the drivers are subjected to harmful vibrations.

[0002] The increased speeds do not only mean increased damages atcollisions but also mean that possible unevennesses in the floor, thatat traditional speed are not noted at all, are transformed into shocksthat in the long run may become disturbing and tiresome for the driver.These shocks may also lead to the transported goods “shaking orbouncing” off from the loading pallet. Since the support wheels of thetruck bounce on the floor a reduction in traction result that in someinstances may make the truck more difficult to steer. Trucks that areused today lack a separate spring system. The only damping component isthe coating of the wheels. With the increasing demand on speed one hasto reduce the “damping” coating of the wheels since a high damping and ahigh speed will result in a too high heat generation for the wheels.This means that the development is on its way towards a situation wheredamages on trucks, goods and drivers will increase since the dynamicstrains increase with the speed at the same time as future demands ofeven larger loads can be expected to make it all even worse.

[0003] The object of the invention is to eliminate or at least reducedthese problems. In accordance with the invention the above problems aresolved by providing the fork truck with a spring system (in addition tothat, that may possibly exist in the coating of the wheels).

[0004] In general this type of trucks have small support wheels or rollsarranged in the front ends of the forks, which wheels due to their smalldiameter are particularly sensitive to unevenness in the floor. It istherefor particularly desirable to provide these wheels with a springsystem and one can therefor possibly be satisfied with providing onlythese wheels with a spring system.

[0005] For the man skilled in the art of trucks a spring arrangement forthe wheels is not logical since the truck at a possible off centeredload preferably is not to become inclined since it then more easily loseits cargo or becomes unstable. Since furthermore the load of a truck canvary with several tons the spring movement will as is realized either beto long or the springing to hard, in particular when the truck is to bedriven empty or with only a small load. In accordance with a further ofdevelopment of the invention this problem is however solved by makingthe spring system load dependent, for instance by a pretensioning of theworking spring device that is proportional to the weight of the load.When a small rock or other protruding unevenness is hit by a springsuspended truck wheel the pretension is overcome by the temporarydynamic additional load and the wheel spring up and reduce in this waythe shock that is a result of the unevenness.

[0006] Additional advantageous characteristics as well as advantages ofthe invention are apparent from the patent claims as well as from thefollowing description of a preferred embodiment. In the drawings FIG. 1shows a schematic sketch of an ordinary truck type provided with adevice in accordance with the invention and with the forks in elevatedposition and FIG. 2 the same truck in a lowered position as to the forksand FIG. 3 a device in accordance with the invention in more detailincluding an accompanying hydraulic coupling scheme.

[0007] The truck shown in FIGS. 1 and 2 includes two forward extendingforks 1, a fork back 2 or wall extending upwards in the rear end of theforks and at last behind the fork back a platform part 3 connected tothis for the driver to stand or sit on. In the lower position shown inFIG. 2 the load forks 1 are lowered and in this position the truck maybe driven in under a loading pallet that is to be lifted and moved or bebacked out from an already moved and lowered loading pallet.

[0008] The platform 3 is in a not shown way journaled verticallymoveable in relation to the fork back 2 and the load forks 1. When theload forks are to be lifted an hydraulic cylinder 4 in the fork back 2push the platform 3 downwards the platform 3. In this way the rear endof the load forks are lifted up from the floor together with the forkback 2. The relative movement between fork back and platform is usedalso to push support wheel pairs 5 in the front ends of the forks downagainst the floor so that the load forks in their front end are liftedas much as in the rear end. The mechanic for the movement of the supportwheels 5 includes a swing arm 6 journaled in the fork back 2, whichswing arm 6 in its front end is pushed down by a spring device 7 that inturn is journaled in the platform part 3. The swing arm 6 transfer viaits swing axle 8 and a second swing arm 9 (movement and force) to a pullrod 10 that in turn pivots a swing arm 11 arranged further to the frontunder the forks and that pivots the support wheels 5 down towards thefloor or ground. For each fork a pull rod with accompanying swing armsand wheels are arranged.

[0009] The spring element 7 has as its object to reduce possible shocksthat occur when the front support wheels (those below the load forks) ofthe truck pass an unevenness. The occurring forces and movements of thesupport wheels 5 are transferred via the above described arms and pullrods to the spring device 7. Since only one common spring device 7 isarranged for the support wheels of the two forks 1 the springing will initself not bring any risk of increased inclination for the truck and therolls on this do not become individually suspended but move springinglyrather in parallel even if only the wheel pair of one fork hit anobstacle.

[0010] Since the rear wheel 12 of the truck is so large in relation tothe small ones the need of springing for this wheel is considerably lessand the essential is thus that the small fronts support wheels 5 becomespring suspended.

[0011] The spring element 7 is designed in the way that is shown in FIG.3 and includes a cylinder 13. A piston 14 inside the cylinder is notfastened directly to the tube-shaped push rod 15 extending out from thecylinder but is resiliently (cup springs 16) supported against a ringshaped element 17 arranged in the inner end of the push rod 15. In orderto guide the piston 14 a piston rod like guiding rod 18 is fastened tothis and is displaceably journaled inside the tube shaped push rod 15extending out of the cylinder 13.

[0012] The cylinder 13 has in the end facing away from the push rod 15 aconnection 20 to the hydraulic system of the truck, coupled in parallelwith the cylinder 4 lifting the forks. This means that when a load islifted from the ground on the forks 1 the pressure that is required forthe lifting cylinder will also be fed to the cylinder 13 in the springdevice 7. This means in turn that the piston 14 in the cylinder 13 ispushed against the cup-springs 16 in proportion to the weight of theload. This compression of the springs does however not result in anychange of position for the extending push rod that the all the time willbe located in the maximally extended position with the ring shapedelements 17 of the push rod 15 in contact with the end 19 of thecylinder 13. Only the pretension is changed.

[0013] When a pressure shock occur due to a support wheel strikingagainst an evenness in the ground or floor the springing with the cupsprings 16 can always take place provided that the pretension force ofthese is overcome. The fact that an extending can not take place doesprevent the support wheels from springing down in holes but mean at thesame time a guaranteed damping of otherwise possible spring oscillationsand a considerable improved comfort and “road holding” is obtained witha truck equipped in accordance with the invention.

[0014] If so desired one could consider to provide the cylinder shown inFIG. 3 with a cup spring or two under the ring shaped element 17 in theinner end of the push rod so that a certain elongation becomes possible.In this way also the shock that could arise when springing has takenplace is dampened. Since furthermore the spring element in this way willbe able to extend somewhat it will be possible for the wheels to springdown into holes. Since the choice of the number of cup springs over andunder the ring shaped element of the push rod respectively the availablespring distance can be adapted to different trucks or conditions.

[0015] The Spring device 7 is coupled to the lifting cylinder by areturn valve 22 that is coupled in parallel with a narrow and restrictedpassage 23. The return valve is oriented so that it allows a rapid flowof hydraulic oil to the spring device 7 at the lifting of a load so thatpractically instantaneously can the pretensioning oil pressure be fed tothe spring device and pretension the springs. At a shock from the wheelsthat is transferred to the push rod 17 and further via the springs 16 tothe piston 14 the oil is not pushed away since the lifting cylindernormally is fully extended against its mechanical end stop, resulting inthe springs taking the shock. Should for some reason the lifting pistonnot be in its maximally extended position the oil however has not timeenough to pass away via the restriction 23 why also in this case themovement is taken by the cup springs 16.

[0016] When the load is once again lowered and put down on the floor theoil pressure in the system vanishes and the oil is led away from thespring device via the restriction and the cup springs expands until thenext lifting takes place. The restriction is chosen so that thecomparatively small amount of oil in the spring device by the cupsprings have time to be pushed out before the truck has backed out fromthe first load and in under the next even if these are locatedimmediately next to each other. The remaining part of the hydraulicsystem of the truck has been denoted 24.

[0017] If the big wheel of the truck should hit an obstacle this willinstantaneously result in an increase in the pressure of the oil thatact on the cylinder and this increase in pressure may in turn mean thatoil is fed away from the cylinder via the return valve (in its forwarddirection) and to the cylinder 13, in which the piston 14 can moveagainst the spring force of the cup springs. Since the piston 14 is notcoupled to the extending push rod no movement or force is transferred tothe front support wheels 5 in the shown case since the push rod alreadyis maximally extended. A certain springing is thus obtained also for thebig wheel of the truck.

[0018] Since at the return springing the oil only flows through therestricted shunting past the return valve this movement is dampened andthere is no risk of the wheel getting into oscillations.

[0019] The restriction also serves as a damping for possibleoscillations between spring device and lifting piston, for instanceduring the lifting itself.

[0020] If required a damping can also be arranged in the spring deviceitself, for instance by a restricted connection through the piston toits other side. Such a connection is in FIG. 3 schematically shown anddenoted 25.

[0021] Above has been described the use of one single spring element,however one can consider to use several spring elements to provide moreindividual springing of separate wheels or groups of wheels.

[0022] As is apparent from the above the shown device enables springingof all the wheels of the truck improving comfort as well as road holdingfor this, simultaneously as no risk exist that an uneven load may makethe truck incline. Nor does different loads result in different heightsof the truck or the forks but this is always the same independent of theweight of the load.

1. Device at low lifting fork truck of the kind that includes supportingwheels or supporting rollers under the forks, which support wheels atthe lifting of the forks execute a moment downwards from the forks,characterized in that in the mechanical movement transfer mechanism oneor several spring elements are arranged for the supporting wheels underthe forks.
 2. Device according to claim 1, characterized in that acommon spring element is arranged for the support wheels of the forks.3. Device according to claim 1 or 2, characterized in that the springdevice is pretensioned corresponding to the weight of the load. 4.Device according to claim 3, characterized in that the pretension isachieved hydraulically with use of the lifting pressure of the truck. 5.Device according to any of the preceding claims, characterized in thatthe spring device has no or only a small resilience in the extendingdirection.
 6. Device according to claim 4 or 5, characterized in thatthe spring device includes an hydraulic cylinder, the piston of whichinfluence a spring element that in its other end influences the bracketof the spring device in the other end, at which the spring deviceincludes means limiting its total length so that a feeding of oil to thehydraulic cylinder pretensions the spring element resulting in thespring device not springing under a static load but only when due tounevenness in floor or ground additional dynamic forces occur, which inthis way can be dampened.
 7. Device according to claim 5, characterizedin that the spring element is arranged inside the hydraulic cylinderbetween the piston and an extending push rod.
 8. Device according 6 or7, characterized in that the spring element with limited degree orcompressibility is arranged between the extending push rod and thecylinder end in push rod end.
 9. Device according to claim 6-8,characterized in that the spring element is constituted by one orseveral cup springs.
 10. Device according to any of the claims 4-9,characterized in that the hydraulic cylinder of the spring device iscoupled together with the lifting hydraulic cylinder or system of thetruck via a return valve that allows feeding of oil to the cylinder ofthe spring device while a restricted connection is arranged in parallelwith the return valve for the return feed back from this when the loadis put down.
 11. Spring device for low lifting truck, characterized inthat it in a first end is constituted by a hydraulic cylinder, thepiston of which influence a spring element that in its other endinfluence the fastening bracket of the spring device in the other end,at which the spring device include means limiting the total length ofthe element so that a feeding of oil to the hydraulic cylinderpretensions the spring element, resulting in the spring device notspringing at static load but only when due to unevenness in the groundor floor additional dynamic forces and movements occur, which in thisway can be absorbed and dampened respectively.